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Self-Assembled Nanorod Structures on Nanofibers for Textile Electrochemical Capacitor Electrodes with Intrinsic Tactile Sensing Capabilities HaoTian Harvey Shi, Nazanin Khalili, Taylor Morrison, and Hani Naguib ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03779 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018

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ACS Applied Materials & Interfaces

Self-Assembled Nanorod Structures on Nanofibers for Textile Electrochemical Capacitor Electrodes with Intrinsic Tactile Sensing Capabilities HaoTian H. Shi†, Nazanin Khalili†, Taylor Morrison†, and Hani. E. Naguib†,‡,* †

Department of Mechanical Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, M5S 3G8, Canada; ‡

Department of Materials Science & Engineering, 27 King's College Circle, Toronto, Ontario, M5S 1A1, Canada; Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, Ontario, M5S 3G9, Canada; *

Corresponding author. Email Address: [email protected]

KEYWORDS: Textile supercapacitor, piezoresistive sensing, nanorod structures, conducting polymers, multi-functional energy storage

ABSTRACT: A novel polyaniline nanorod (PAniNR) three-dimensional structure was successfully grown on flexible polyacrylonitrile (PAN) nanofiber substrate as the electrode material for electrochemical capacitors, constructed via self-stabilized dispersion polymerization process. The electrode offered desired mechanical properties such as flexibility and bendability, while it maintained optimal electrochemical characteristics. The electrode and the assembled electrochemical capacitor cell also achieved intrinsic piezoresistive sensing properties, leading to the real-time monitoring of excess mechanical pressure and bending during cell operations. The PAniNR@PAN electrodes show an average diameter of 173.6 nm, with the PAniNR growth of 50.7 nm in length. Compared to the electrodes made from pristine PAni, the gravimetric capacitance increased by 39.8% to 629.6 F/g with aqueous acidic electrolyte. The electrode and the assembled EC cell with gel electrolyte was responsive to tensile, compressive, and bending stresses, with a sensitivity of 0.95 MPa-1.

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1. INTRODUCTION The field of wearable and flexible electronics has received substantial scientific interests in the recent years, mainly due to the rapidly growing demand for real-time health monitoring and personal textile electronic devices.1-2 To supply the necessary energy to ensure their proper functioning, flexible, yet lightweight energy storage systems, such as thin-film textile supercapacitors or batteries, must be utilized.3-5 Supercapacitors or electrochemical capacitors (ECs) belong to an emerging class of energy storage systems that can deliver high power densities while maintaining the desirable energy characteristics. 6-8 Many of the currently studied flexible EC electrodes are realized by coating active pseudocapacitive materials onto flexible substrates such as plastic and thin metallic films. 9-10 However, with no significant improvement in specific active surface areas, the electrodes’ capacitive potential was not fully realized. Researchers have also focused on other physical forms of flexible electrodes, such as hydrogel and aerogels to ensure the surface area contact would not be jeopardized due to electrode flexing, but the gel-type electrodes typically lead to increased cell thickness, which is undesired for wearable devices.11-15 Various EC electrode geometries were also designed for improved suitability for wearable smart textiles.16-18 Guo et al. investigated the possibility of utilizing double-helical wire geometries for the electrode systems utilizing titanium@MnO 2 structures showing an operating voltage of 0.8 V and an areal capacitance of 15.6 mF cm–2.19 With the novelty in geometry, the EC cell performed well under different cyclic loading conditions, however, the material’s inherent capacitance was not as high as desired. Additional literature have placed emphasis on the fabrication of textile yarn electrodes, embedded directly into clothing and other flexible substrates.20-21 Li et al. investigated the potential to utilize a core-shell structure with a framework consisting of carbon nanotube (CNT) sponges with polypyrrole networks.22 However, due to the nature of the CNT sponge network, the EC suffered from lack of 2 ACS Paragon Plus Environment

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stretchability and bendability. It has become clear that the utilization of simple substrate for deposition of nanostructures would not be sufficient to improve the overall charge storage performance and therefore, a hierarchical yarn-like EC electrode with high specific active surface area and mesoporous three-dimensional structures is needed for textile applications. 23-24 In our work, electrospinning was utilized as the manufacturing method to create desirable mesoporous flexible substrate, with ultrasound-assisted self-assembly of highly adhered active surface nanoarchitectures, leading to improved flexibility, while maintaining its energy storage capabilities. Currently available energy storage devices are fabricated with bulky and hard casings that protects the user from any potential cell shorting or electrolyte leakage. Flexible ECs are designed without such casings, and therefore it is imperative to minimize the applied stress during cell functioning to prevent such catastrophic outcomes. If an EC cell is capable of sensing mechanically applied compressive, tensile, and bending loads, possible device failures can be avoided with real-time detection. The two main sensing mechanisms that can be practically implemented in wearable applications are resistive and capacitive types. These two mechanisms require less complex measurement equipment and offer higher material flexibility compared to other types of sensors such as piezoelectric and fiber Bragg grating.25-26 However, due to the inherent limitations of capacitive-type sensors, their sensitivity is generally lower than that of the resistive ones.27 The main mechanism driving the piezoresistive effect resulting in contact resistance changes is the structural deformation of the sensing element. The change in contact resistance allows more tunable properties and has been widely used within different structures as flexible

strain

and

pressure

sensors

including

fibrous

substrates, 28-29

elastomeric

nanocomposites,30-31 and conductive hydrogels.32-33. Even though it has become standard practice to utilize high angle mechanical bending tests to study the degradation of ECs with rigorous bending or stretching, past studies seldom addressed 3 ACS Paragon Plus Environment


the prevention of excessive bending and stretching during EC operations. 19,

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34-35

Taking

advantage of the piezoresistive sensing effect based on the change in the contact area of the asfabricated EC electrodes would enable EC cells with additional tactile sensing functionalities. Herein, a novel yarn-type EC electrode with hierarchical design, utilizing the nanoscale architectures of PAni nanorods (PAniNR), formed with ultrasound-assisted self-assembled polyaniline (PAni) on nanofibrous electrospun polyacrylonitrile (PAN) has been investigated. PAniNR@PAN, a fully flexible textile yarn-like hierarchical, nanostructured electrode, as shown in Figure S1, was constructed and evaluated. The PAniNR@PAN electrodes showed an average diameter of 173.6 nm, with the PAniNR growth of 50.7 nm in length and 45.6 nm in diameter. It was found that compared to the powder electrodes with pristine PAni, the gravimetric capacitance increased by 39.8% to 629.6 F/g resulting from an enhanced electrode/electrolyte interface. The nanofiber electrodes were further tested with both mechanical bending and stretching, resulting in no noticeable performance deformations. Galvanostatic charge/discharge cycling tests resulted in only 12.4% decrease after 3000 cycles, which is superior than many other pseudocapacitive electrodes. The fabricated PAniNR@PAN electrodes and the assembled functioning EC cells were shown to possess real-time piezoresistive sensing capabilities due to their unique multilayered 3-dimensional structures. The electrode was responsive to both tensile and compressive stresses as well as bending, attaining a maximum sensitivity of 0.95 MPa -1. The resistance output from the assembled EC cell was also highly responsive to finger tapping and bending, with fast structural recovery times. To the best of the authors’ knowledge this study is the first time reporting an EC cell with real-time sensing capabilities utilizing the intrinsic piezoresistive properties of the PAniNR@PAN electrode system.


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2. EXPERIMENTAL 2.1 Materials and Instrumentation Ammonium persulfate (APS), polyacrylonitrile (PAN M w=150,000), dimethylformamide (DMF), and polyvinyl alcohol (PVA Mw=135,000) were obtained from Sigma-Aldrich and used without further modification. The aniline monomer was also obtained from Sigma-Aldrich and underwent steam distillation prior to use. The hydrochloric acid (HCl) and sulphuric acid (H 2SO4) were both obtained from Caledon Laboratory Chemicals. The sonication process was assisted with Qsonica Q700 Sonicator at an amplitude of 15. Electrochemical characterization was carried out using a CH Instruments 6054E electrochemistry workstation. Scanning electron microscopy was performed with an FEI Quanta FEG 250 Environmental FE-SEM. AFM height maps were obtained with a Bruker Multi-Mode 8 system in tapping mode. X-ray diffraction (XRD) was carried out using a Philips XRD analyzer with a PW 1830 HT generator, a PW 1050 goniometer between 2θ values 5° and 35°. Absorption Fourier transform infrared spectroscopy (FTIR) was conducted with a Bruker ALPHA system. Additionally, thermogravimetric analysis (TGA) was performed with a TA Instruments model Q50 analyzer, between 25°C and 900°C with a temperature ramp rate of 10°C/min. The piezoresistive sensing capabilities of the electrode and the assembled EC cell were simultaneously measured using a Keithley 2400 sourcemeter and an Instron testing system in the compression mode, shown in Figure 1. By using the sourcemeter to monitor the resistance change, while allowing the Instron to apply cyclic stress loadings to the sample surface, the piezoresistive responses can be analyzed. The sourcemeter was also used to measure the resistance change during tapping and bending.



Figure 1. To monitor the PAniNR@PAN nanofiber electrode piezoelectric responses to externally applied stress, the sample is secured on the Instron testing platform and apply cyclic loading repeatedly to the sample surface, while using the sourcemeter to monitor the resistance change during the pressure change.

2.2. Fabrication Method The PAni powder electrodes were made via a conventional assembly route, utilizing thin stainless-steel shim as current collectors. 2 mmol of aniline monomer was first added to 10 mL of distilled water, and sonicated for 30 seconds. This sonication process led to a cloudy colloid of aniline monomer and distilled water. The self-stabilized dispersion polymerization takes place when the aniline solution was added to 10 mL of HCl of varying concentrations, mixed with 2 mmol of APS as the oxidant. In this study, a variation of HCl concentrations were selected, namely, 0M, 0.25M, 0.50M, 0.75M, and 1.0M, for optimizing electrochemical properties. After 4 hours of polymerization at room temperature, the PAni powder synthesized was filtered and dried at 60°C. Then, the powder was weighed to similar quantities as the nanofibers and added to a binder mixture of graphite adhesive ink and polytetrafluoroethylene (PTFE). The paste was then mixed with sonication and brushed onto the thin stainless steel current collector to be used as the electrode. The PAniNR@PAN nanofiber electrodes were made with the electrospun PAN fibers as the substrate, with ultrasound-assisted dispersion polymerization of PAni on the surface, as 6 ACS Paragon Plus Environment

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demonstrated in Figure 2. The as-obtained PAN powder was first dissolved in DMF to obtain a 5 wt.% PAN polymer solution. An appropriate amount of PAN powder was placed in a beaker with DMF and stirred at 80°C overnight to obtain an uniform transparent viscous PAN solution. Then the PAN solution was placed into a 20-mL plastic syringe, and dispensed at 1 mL/Hr rate for 6 hours using a gauge 16 needle with a 20 kV DC tip voltage. Aluminum foil was used as the collector and the collection distance was 10 cm. The PAN electrospun film was then placed into an oven for drying at 60°C. After drying, it was cut into strips for the PAniNR growth process.

Figure 2. Fabrication process schematics for the fabrication of PAniNR@PAN nanofiber textile electrode via selfstabilized dispersion polymerization. The process starts with the conventional electrospinning with 5 wt.% PAN in DMF solution. The PAN film was then placed into a solution of oxidant and HCl to complete wet its surface. Then the polymerization takes place for 12 hours with a measured amount of aniline monomer and distilled water colloid added to the oxidant solution.

The pristine as-spun PAN film was then placed within a solution containing 2 mmol of APS and varying concentrations of HCl. The film can be fully immersed in aqueous solutions due to its hydrophilic nature. Another agitated solution containing 2 mmol of aniline monomer and 10 mL of distilled water was then added dropwise. The ultrasonication induced a formation of turbid colloid liquid containing the aniline monomer. Then, polymerization was allowed for 4 hours at 7 ACS Paragon Plus Environment


room temperature. The film was then removed from the solution and washed repeatedly with ethanol and water to remove any oligomers from the nanofiber surfaces. It was found that the adhesion of the PAniNR on the PAN nanofibers was well-established and it was observed that the PAniNR formations do not wash off from the film surface. The PAniNR@PAN films were then allowed to dry at room temperature for 24 hours. Symmetric ECs were assembled with grade 304 stainless steel shim (McMaster Carr), utilizing carbon adhesive to improve the electrical contact for optimized conductivity. The aqueous electrolyte used for characterization was 1M H 2SO4 aqueous solution and filter paper was used as the separator. The gel electrolyte was fabricated by adding 1g PVA to 10mL distill water, and then heating the solution to 85°C with magnetic stirring at 400 RPM. After the solution becomes transparent and the PVA pellets fully dissolved, 10 mL of 1M H2SO4 was added and further stirred for 1 hour. The solution was then dried at room temperature with a relative humidity of 30% for 48 hours prior to use. The gelled electrolyte was cut into the specified dimensions and sandwiched between the electrodes for twoelectrode characterization purposes. 3. RESULTS & DISCUSSIONS 3.1. PAniNR@PAN Morphology Scanning electron microscopy (SEM) images shown in Figure 3 has revealed that the PAniNR@PAN nanofibers experienced PAni nanorod structure growth on the electrospun PAN nanofiber surface. Figure 3 a), b) have shown that the pristine electrospun PAN films demonstrated a uniform and smooth surface. The PAN electrospun film serves as the flexible substrate that provides the desired mechanical properties for the EC electrode. Figure 3 c) shows a yarn-like electrode structure that can be easily integrated with the woven textile as shown previously. Figure 3 d)-f) shows the magnified images of the PAniNR@PAN nanoarchitecture, demonstrating the growth of PAni nanorods on the PAN fibers. After further analysis as shown in 8 ACS Paragon Plus Environment

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Figure 4 a), it was found that the average diameter for pristine as-spun PAN fibers are 72.2±22.1 nm, while the average diameter increased to 173.6±26.0 nm for the PAniNR@PAN fiber electrodes.

Figure 3. Scanning electron microscope (SEM) images of a) & b): Pristine electrospun PAN films serving as the core substrate for the deposition of the active PAniNR structures on the surface; c)-f) Yarn-like hierarchical PAniNR@PAN electrodes fabricated with the self-stabilized dispersion polymerization process. These PAniNR@PAN structures are fabricated with 1M concentration HCl acidic environment.

Figure 4. a) Histogram of diameter measurements of pristine PAN nanofiber and PAniNR@PAN films, demonstrating an average diameter of 72.2±22.1 nm for pristine PAN nanofibers and 173.6±26.0 nm for PAniNR@PAN fibres. The PAni nanorods showed an average length of 50.7 nm and a diameter of 45.6 nm; b) shows an SEM image showing the pristine PAN nanofibers, and c) shows an SEM image showing the PAniNR@PAN nanostructured fibers; d) & e) show the mechanical flexibility and bendability as demonstrated by the flexing and bending of the PAniNR@PAN electrode.



This shows a growth of nanorods with an average length of 50.7 nm and an average diameter of 45.6 nm. This PAniNR@PAN formation is a result of the selection of appropriate combined aqueous chemical synthesis techniques and parameters. Due to the hydrophilic nature of the pristine PAN films, the APS oxidant solution with the dopant HCl could effectively wet the entire surface of the PAN nanofibrous template to adequately facilitate the PAni polymerization process. To improve the number of active reaction sites for the pseudocapacitance charge storage and better facilitate the ion transport through the electrode’s three-dimensional pores, increased roughness on the nanoscale was also desired. It was shown previously that the rigorous agitation can create a colloid of aniline in the distilled water, with a cloudy appearance, which when added to the oxidant solutions can induce self-stabilized dispersion polymerization of aniline, leading to an improved electrical conductivity, consequently contributing to the overall EC performance. This method allows for an improved growth mechanism of the PAni structures without the undesired growth of oligomers and other side branching effects, leading to more uniform polymerization in the aqueous phase. Additionally, the use of the PAN nanofiber substrate allowed enhanced interactions of the chemical oxidant agent with the aniline monomer at the substrate surface, inducing preferential growth of PAni in a more aligned and uniform fashion. Thus, as shown in Figure 4 c), the resulting formation of the uniform PAniNR structures are due to the combined effect of colloidal formation of nano-sphere encapsulated aniline monomer, as well as the template induced polymer alignment at the PAN substrate surface. Additional SEM micrographs of PAniNR@PAN surfaces and the nanorod formations are provided in Figure S2. The PAniNR@PAN fibrous films retained the flexibility and bendability as observed with pure electrospun PAN films. As shown in Figure 4 d) and e), it was clear that the mechanical flexing and bending do not cause permanent mechanical deformation to the PAniNR@PAN electrode. Dusting is one of the major issues with the powder type electrodes, since it is easy to 10 ACS Paragon Plus Environment

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cause undesired detachment or delamination of active electrode material from the current collector surface, which can lead to premature EC cell failures. However, with rigorous washing and scratching, dusting was not observed in the case of PAniNR@PAN fiber electrodes, owing to the excellent adhesion between the PAniNR and PAN nanofibers. This can be attributed to the hydrophilicity of PAN and the desirable wetting characteristics of PAN electrospun films during fabrication.

Figure 5. Atomic force microscopy (AFM) height images of a)-c): Pristine electrospun PAN fibres, where the measured diameters are consistent as measured with the SEM shown previously; d)-f): PAniNR@PAN fibrous electrodes morphologies showing increased roughness after the PAniNR growth.

Additionally, from the atomic force microscopy (AFM) height images obtained using tapping mode, as shown in Figure 5, the uniform PAni nanorod growth on the PAN nanofiber substrate surface was verified. From a)-c), it is shown that the pristine PAN presents a smooth clean surface, with fiber diameters approximate to that of the as-spun PAN in the SEM analysis. The surface roughness significantly increased as the PAni nanorods layer was introduced, shown in Figure 5 d)-f). It should be noted that the PAniNR@PAN fibrous film electrode retained a mesoporous structural network that allows the electrolytic ions to easily access the active reaction 11 ACS Paragon Plus Environment


sites for efficient charge storage in the EC. The diffusion distance of the electrolyte into the electrode for electrochemical interaction is considered very short and can be anywhere between 10-30 nm.36 The thickness of the coated PAniNR layer was controlled to provide sufficient thickness of electrode active material for charge storage, while still maintaining the porosity to allow efficient ionic movement. It was also hypothesized that PAniNR@PAN electrode nanoscale morphology can be tailored with varying HCl concentrations during polymerization and doping process, which can affect the electrochemical performance of the EC electrode. And therefore, a variety of HCl concentrations were utilized during the dispersion polymerization process. From SEM images in Figure S3, it is possible to observe the effect of varying HCl concentrations during polymerization on the morphology of the PAniNR@PAN fibre nanostructures. It is interesting to note the formation of hollow nanotubular PAni structures with 0M HCl environment, which can lead to increased specific surface areas, however, the PAni doping level will be significantly affected, which can negatively contribute to the energy storage capabilities. The utilization of 0.5M and 1.0M HCl concentrations have proved to assist in the PAniNR formations on the PAN fiber surface, allowing for increased doping levels. The PAniNR@PAN morphology did not vary significant between 0.25M, 0.5M, 0.75M, and 1.0M HCl polymerization environment. 3.2. Compositional Characterization (Moved from 3.4 to here) From the thermogravimetric analysis (TGA), shown in Figure S4 a), it is possible to deduce that the PAniNR active material has accounted for 50.1% of the entire weight of the PAniNR@PAN electrode, which constitutes a significant mass loading percentage. The high active material loading ensured the desired pseudocapacitive behaviours during the electrochemical testing cycles, resulting in the high specific capacitance as observed. Combining 12 ACS Paragon Plus Environment

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with the morphology with the mass loading character of the PAniNR@PAN films, it is possible to hypothesize that the thickness of the PAniNR layer was well-established to facilitate the efficient ion transport, while ensuring sufficiently thick PAni active layers in the range of 20-30 nm were uniformly deposited on the fibre substrate to enable Faradaic interactions at the electrolyte contact. Further X-ray diffraction (XRD) study, shown in Figure S4 b), confirmed the composition of the PAniNR@PAN composite electrode. It has been observed that the peaks shown for PAniNR@PAN fibre electrodes at the at 2θ values 15°, 21°, and 26° corresponds to the (011), (020), and (200) crystal planes.37 Additionally, the board peak at the 16.7° was observed for both the PAniNR@PAN fibres and the pristine PAN fibre. This is consistent with previously reported principal peak for PAN films ranging between 2θ values of 16.4° to 17.2°, corresponding to the equatorial reflections in the PAN structures with a lattice spacing of 5.3 Å. 38 From the Fourier transform infrared spectroscopy (FTIR) shown in Figure S4 c), it is also possible to determine the composition of the PAniNR@PAN electrodes. Pristine PAN films mainly include two distinct peaks in their FTIR spectra. A distinctive peak at 2243 cm -1 is due to nitrile stretch (C≡N), while the peak at 1451 cm -1 is assigned to CH2 bending frequency which, in both cases, were also apparent in the PAniNR@PAN case. 39 Pristine PAni Powder contained peaks at 1571 cm-1 and 1490 cm-1, representing the quinoid ring and benzenoid ring stretching, respectively. Additionally, PAni also shows absorption peaks at 1300 cm -1, representative of C–N stretching of secondary aromatic amine. C–N in the polaron lattice of PAni shows a peak at 1245 cm-1. It is clear from the FTIR that the PAniNR@PAN fibre electrode contained similar peaks as described in both the pristine PAN fibre and the pristine PAni powder cases.



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3.3. Electrochemical Performances From the morphology studies, it was deduced that the HCl concentrations in the polymerization would have a significant contribution to the variations in the specific capacitance values for the PAni active layer. Herein, the gravimetric specific capacitances are calculated using the active PAni materials on the electrode without the considerations of the substrate in the case of PAniNR@PAN or binders in the case of PAni powder electrodes. Specifically, in this two-electrode case of symmetric EC cell, the specific capacitance of the capacitor is determined by the formation of equivalent circuit connected in series, which means that the assembled EC capacitance is half of that of the electrode. With the consideration of the CV diagrams, the specific capacitance was calculated as shown in Equation (1):

Cs 

2 S  w  V



vf

vi

i  VdV

(1)

Where S is the CV scan rate in V/s; w is the mass loading of the active material in g; ΔV is the voltage window in V, i is the measured current in A, and V is the voltage in V. As shown in Figure 6 a), it was found that the capacitance for PAni powder electrode fabricated with 0M HCl environment was significantly lower than that of PAniNR@PAN fibre electrodes fabricated with the same aqueous conditions. This can be attributed to the formation of hollow nanotubular morphologies of PAni which enabled further doping when the aqueous H2SO4 electrolyte was applied on the electrode surface during cell assembly. The PAni nanotube’s large surface area also contributed to a larger contact at the electrolyte and electrode interface, allowing higher utilization of the PAni active materials, and therefore further contributing to the specific capacitance. PAni powder electrode fabricated at 0M HCl, on the other hand, did not experience sufficient doping and therefore after the carbon adhesive and


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PTFE binders were applied, the electrochemical activity further dwindles and cannot effectively contribute to the overall capacitance.

a)

b)

c)

d)

Figure 6. a) Cyclic voltammograms at 10 mV/s of PAniNR@PAN fibre electrodes with varying HCl environment during the polymerization process, with additional comparison to the pure PAN fiber electrode; b) A comparison of gravimetric specific capacitance values of PAniNR@PAN fibre electrode to that of the PAni powder electrodes, with variations in HCl concentrations; both of which are measured at 10 mV/s scan rates; c) Electrochemical impedance spectroscopy (EIS) of the PAniNR@PAN electrodes fabricated at different HCl conditions; d) A comparison of PAniNR@PAN fibre electrode rate capabilities.

From Figure 6 b), it is demonstrated that the formation of the PAniNR structures on the PAN nanofiber substrate allowed for increased reaction sites to interact with the electrolytic ions and therefore contributed to a substantial increase in gravimetric specific capacitance in comparison to the conventional PAni powder electrodes fabricated with the same aqueous conditions. From the same figure, it is also interesting to note that, unlike the PAni powder electrodes, where the capacitance increased with increasing concentrations of HCl, the PAniNR@PAN fibre electrodes experienced a sharp drop in specific capacitance at 0.5M concentrations of HCl. This can be explained by the electrochemical impedances as revealed in Figure 6 c). Both the series resistance 15 ACS Paragon Plus Environment


(Rs) and charge transfer resistance (Rct) for 0.5M HCl case were higher at 7 Ω and 14.7 Ω, respectively, as compared to PAniNR@PAN fibres made with other concentrations of HCl. Even though in the morphology analysis, no significant differences were noted between 0.25M to 1.0M HCl cases, there are resistance that was introduced, which contributed to the impedance of charge transfer at the electrode/electrolyte interface, contributing to a noticeable decrease in the specific capacitance. PAniNR@PAN produced with 0.25M and 1.0M HCl both experienced desirable electrochemical characteristics, which contributed to similarly high specific capacitances of 623.2 F/g and 629.6 F/g, respectively. However, when comparing the rate capabilities of these two conditions, it was observed that the PAniNR@PAN fibre electrodes fabricated with 1.0M HCl acidic conditions performed noticeably better at higher scan rates as shown in Figure 6 d). This can be further explained by the differences in charge transfer resistances of the two cases, where the 1.0M HCl environment allowed for lower charge transfer, almost half as that of for 0.25M HCl. Therefore 1.0M HCl has been selected as the fabrication condition used for further PAniNR@PAN fibre electrode characterization.

Figure 7. Electrochemical analysis and characterization of PAniNR@PAN nanofibers using 1M H2SO4 aqueous electrolyte, fabricated with 1M HCl acidic environment: a) The variation in CV diagrams with different scanning rates from 5mV/s to 100mV/s; b) Galvanostatic charge/discharge curves; and c) Galvanostatic charge/discharge cycling capabilities of the PAniNR@PAN nanofiber structures.

As shown in Figure 7 a), the symmetrical nature of the cyclic voltammogram and its reversibility demonstrated the fact that the assembled symmetric EC cell utilizing 16 ACS Paragon Plus Environment

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PAniNR@PAN electrodes can be effectively employed for capacitive energy storage purposes. Additionally, the symmetrical CV demonstrated that the capacitive behaviour is not diffusion controlled, confirming the prior hypothesis that the charge transfer impedance introduced by the three-dimensional electrode nanostructured network was not large enough to cause significant IR drops during charging and discharging. The REDOX peaks shown in the CV graph also confirmed the pseudocapacitive nature of the active material, giving rise to the Faradaic charge transfer that takes place at the PAniNR@PAN electrode/electrolyte interface. Measured at 10 mV/s scan rates, the gravimetric capacitance for the PAniNR@PAN electrode is 629.6 F/g, corresponding to an energy density of 63.0 Wh/kg. The rate capabilities of the PAniNR@PAN electrodes were also characterized. At higher scan rates, the CV graphs are somewhat skewed and therefore yielded a lower capacitance in comparison to the slower scan rates. The galvanostatic charge discharge (GCD) curves for the PAniNR@PAN fibre electrodes, as shown in Figure 7 b), shows minimal IR drops during the discharge cycle, also further verifying the specific capacitance values measured with cyclic voltammetry. Lastly, the GCD cycling test in Figure 7 c) shows that after 3000 cycles, the PAniNR@PAN electrodes retained 551.5 F/g of capacitance, which constitutes a 12.4% decrease compared to the initial capacitance value of 629.6 F/g. This decrease is significantly smaller as compared to previously reported pseudocapacitive ECs. With the utilization of the PVA-H2SO4 gel electrolyte, the two-electrode EC cell testing was carried out to ensure that the proposed sensing mechanism is not affected by the electrolyte resistance and can occur simultaneously with the storage of charge. Figure S5 a) shows that the cyclic voltammetry diagram of the two-electrode test cell is indicative of symmetrical charge/discharge capacitances. Figure S5 b) showed the observed R s of 3.75 Ω, and a Rct of 0.91 Ω overall, indicative of good compatibility between the PVA-H2SO4 gel electrolyte and the



PAniNR@PAN electrode system, which ensures that the usage of gel type electrolyte can be effectively utilized without sacrificing the capacitance of the flexible PAniNR@PAN electrodes. 3.4. Tactile Sensing

Figure 8. Sensing capability of the PAniNR@PAN electrode and EC cell: a) Piezoresistive response of PAniNR@PAN nanofibers to compressive stress with a constant applied rate (inset: resistance change in response to tensile stress); b) Piezoresistive response of PAniNR@PAN nanofibers to bending; c) Piezoresistive response of the EC cell to compressive stress with a constant applied rate; d) Time response of the nanofibers to compressive stress indicating a linear behaviour; e) Piezoresistive response of the nanofibers to compressive cyclic loading; and f) Schematic of the sensing mechanism of the nanofibers.

The PAniNR@PAN electrode and the EC cell were exposed to compressive, tensile, bending and cyclic loadings with their electrical resistance measured in real-time. They both exhibited substantial piezoresistive response due to the applied structural deformation which reveals that the PAniNR@PAN electrode acts as the main sensing element within the structure of the EC cell. Figure 8 a) and b) depict the piezoresistive response of the EC electrode to compressive and bending loadings, respectively. The full EC cell also responded to a mechanically applied compressive stress with a constant rate as shown in Figure 8 c). It should be noted that both the electrode and the EC cell showed linearity in their response to the applied load which is a 18 ACS Paragon Plus Environment

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considerable aspect for calibration purposes. The same linear behaviour was observed in the response of the PAniNR@PAN electrode to tensile loading (inset of Figure 8 a). Furthermore, the time/piezoresistive response of the electrode under compressive loading as depicted in Figure 8 d) confirms its linearity. The electrode was also tested under cyclic compressive loading and as shown in Figure 8 e), it offers small hysteresis behaviour exposed to dynamic loadings. The proposed mechanism of the piezoresistive response of the PAniNR@PAN electrode is schematically portrayed in Figure 8 f). With an applied mechanical stress, the layers of soft PAniNR@PAN fibers move closer together, contributing to an increase in the number of contact points of the individual conductive PAniNR coated PAN fibers. And this increase ultimately leads to lower electrical resistance, as the network of PAniNR@PAN fibers contains more electrically connected conduction points. It should also be noted that the elastic nature of the electrospun nanofibers contributes to reproducibility of the resistance response of the PAniNR@PAN sensor after repeated loading-unloading cycles. Additional pressure sensing tests involving fully functional EC cells took place with finger tapping actions. As shown in Figure 9, the current collector was purposely separated into two pieces and the connection was made with the sensing electrode to monitor the resistance changes during tapping. The purpose of this design is to directly monitor the resistance across the electrode material and to ensure that the measurement is not affected by the electrolyte resistance. The resistance measurement is shown in Figure 9 a), confirming the hypothesis that the during normal EC cell operations, the application of mechanical stresses, such as the tapping pressure of finger tips, can be readily observed via the piezoresistive responses of the PAniNR@PAN electrodes. The pressure applied by the finger tip translates to a compressive stress applied onto the fiber, leading to the formation of a more conductive network with an increased contact points, resulting in a lower observed resistance. 19 ACS Paragon Plus Environment


Figure 9. a) The piezoresistive response of the PAniNR@PAN during finger tapping of the fully functioning EC cell, the peaks are increases in the resistance measurements that correspond to the tapping stresses; b) The setup used for when monitoring the piezoresistive response of the fully assembled EC cell, note the gap in the current collector on the sensing electrode side, which is used to expose the electrode material to the applied pressure; c) The measurement setup used for this test, which includes a sourcemeter for monitoring.

It is important to note that the recovery of the electrode resistance is a time-dependent phenomenon, which is attributed to the compressive strain recovery of the PAniNR@PAN electrode layer. After it has returned to its original conformation, the measured resistance returns to its original value. When there is constant pressure applied, the measured resistance values are also constant, signifying that the resulting compressed electrode structure yielded similar conformations, resulting in the formation of conducting network structures with similar connections. The bending of the flexible EC can also be effectively monitored with this inherent PAniNR@PAN electrode piezoresistive sensing mechanism. Following repeated flexing of the EC cell between a bending angle of 0° and 30°, the resistance changes are as demonstrated by 20 ACS Paragon Plus Environment

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Figure 10 a). It can be recognized that the higher resistances correspond to the flat non-bent states, while the lower resistance values are related to the bent state. The comparison between the bent and flat states is shown in Figure 10 b). The resistance curve shows an overshoot during the recovery from the bent states, indicating that there are residual conformation changes following the recovery, leading to a resistance overshoot. However, this phenomenon diminishes with time and the PAniNR@PAN electrode resistance can eventually return to its original value.

Figure 10. a) The resistance change resulting from mechanical bending of the PAniNR@PAN EC cell, arrows showing the time instants when the bending occurred; b) A comparison for the EC cell in a bent state vs. in a flat non-bent state.

The piezoresistive sensing ability of the PAniNR@PAN films are comparable to thin films piezoresistive sensors with similar thicknesses. Previous studies on electrospun pressure sensors for health monitoring applications reported a polyvinylidene fluoride (PVDF) fiber mat with reduced graphene oxide (rGO) coating as the active layer.40 Their obtained electric current change in response to pressure, bending and torsion was within the same range as our reported 21 ACS Paragon Plus Environment


sensor. A similar sensing technique was used via gold-nanowire coated tissue paper with microscale fibers.41 The sensor was also responsive to different types of applied mechanical forces with the same detection range. In another study, polypyrrole coated PVDF fibers were electrospun and used as pressure sensors with a high electrical resistivity drop for an applied pressure range of 5 MPa.42 Although their reported resistivity change is higher than our proposed sensor, their limit of detection (LOD) is much larger than that of appropriate for regular sensing applications. It should be noted that due to the thin form factor, high flexibility, stretchability of the thin-film supercapacitor, as well as the integration requirements for the electrolyte and electrode interface, it is the best sensing performance that can be achieved with simultaneous energy storage. This information provided by simultaneous sensing can be used as early warning signals for users so that once excessive stress is observed, appropriate actions can be taken to ensure the proper functioning of the EC cell and prevent any premature failures. One of the major benefits of this design is that it is no longer necessary for external sensing mechanisms as the sensing element is fully integrated as an inherent material’s property of the PAniNR@PAN electrode. 4. CONCLUSIONS In this work, a novel PAniNR@PAN EC electrode was successfully constructed via selfstabilized dispersion polymerization of PAni with nanofibrous electrospun PAN flexible substrate. With its superior flexibility and bendability, the reported EC electrode is capable of repeated bending and flexing without sacrificing charge storage capabilities. Morphological studies revealed the hierarchical, nanostructured nature of the fibre electrode with desirable PAni nanorod growth on the PAN surface. PAni grown in a nanorod-like nature for an average length of 50.7nm, corresponding to a desired thickness for electrolytic ion interactions. It was also found that compared to the conventional pristine PAni powder electrodes, the gravimetric capacitance 22 ACS Paragon Plus Environment

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was measured to be 629.6 F/g, representing a 39.8% increase. The energy density of the PAniNR@PAN electrode was calculated to be 63 Wh/kg of electrode material. Additionally, only 12.4% decrease in capacitance was observed after 3000 charge discharge cycles. The sensing capabilities of the electrode and EC cell was also demonstrated with considerable linear response to compressive, tensile, bending, and dynamic loading conditions. This unique feature enables the device to prevent potential failures due to high mechanical deformations. This PAniNR@PAN novel hierarchical nanostructured electrode not only allows for easy integration into textile substrates to supply energy required for wearable actuators, transmitters, and displays, but also serves as a tactile sensor element with its inherent piezoresistive sensing properties.

SUPPORTING INFORMATION 

Figure S1: Schematic Representation of hierarchical textile yarn EC fabricated with PAniNR@PAN nanofibers

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Figure S2: SEM images of PAniNR@PAN fibre fabricated demonstrating nanorod formation

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Figure S3: SEM images of PAniNR@PAN fibre film fabricated with various HCl concentrations during fabrication for morphological comparison

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Figure S4: Compositional analysis PAniNR@PAN nanofibers: thermogravimetric TGA, Xray diffraction (XRD), and Fourier-Transform Infrared Spectroscopy (FTIR)



Figure S5: Electrochemical analysis with PAniNR@PAN using PVA-H 2SO4 gel electrolyte

AUTHOR INFORMATION Corresponding Author *Email: [email protected]



ACKNOWLEDGMENT The authors would like to acknowledge Natural Sciences and Engineering Research Council of Canada (NSERC) (Grant# 459389), Canada Foundation for Innovation (CFI) (Grant# 481796), and Canada Research Chairs program (CRC) (Grant# 480255) for the financial support they have provided for this research work.

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THE TABLE OF CONTENTS KEYWORDS: Textile supercapacitor, nanorod structures, conducting polymers, piezoresistive sensing, pseudocapacitive energy storage TITLE: Self-Assembled Nanorod Structures on Nanofibers for Textile Electrochemical Capacitor Electrodes with Intrinsic Tactile Sensing Capabilities AUTHORS: HaoTian H. Shi, Nazanin Khalili, Taylor Morrison, and Hani. E. Naguib*



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To monitor the PAniNR@PAN nanofiber electrode piezoelectric responses to externally applied stress, the sample is secured on the Instron testing platform and apply cyclic loading repeatedly to the sample surface, while using the sourcemeter to monitor the resistance change during the pressure change. 907x522mm (96 x 96 DPI)



Fabrication process schematics for the fabrication of PAniNR@PAN nanofiber textile electrode via selfstabilized dispersion polymerization. The process starts with the conventional electrospinning with 5 wt.% PAN in DMF solution. The PAN film was then placed into a solution of oxidant and HCl to complete wet its surface. Then the polymerization takes place for 12 hours with a measured amount of aniline monomer and distilled water colloid added to the oxidant solution. 2191x1243mm (96 x 96 DPI)


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Scanning electron microscope (SEM) images of a) & b): Pristine electrospun PAN films serving as the core substrate for the deposition of the active PAniNR structures on the surface; c)-f) Yarn-like hierarchical PAniNR@PAN electrodes fabricated with the self-stabilized dispersion polymerization process. These PAniNR@PAN structures are fabricated with 1M concentration HCl acidic environment. 1873x921mm (96 x 96 DPI)



a) Histogram of diameter measurements of pristine PAN nanofiber and PAniNR@PAN films, demonstrating an average diameter of 72.2±22.1 nm for pristine PAN nanofibers and 173.6±26.0 nm for PAniNR@PAN fibres. The PAni nanorods showed an average length of 50.7 nm and a diameter of 45.6 nm; b) shows an SEM image showing the pristine PAN nanofibers, and c) shows an SEM image showing the PAniNR@PAN nanostructured fibers; d) & e) show the mechanical flexibility and bendability as demonstrated by the flexing and bending of the PAniNR@PAN electrode. 2050x866mm (96 x 96 DPI)


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Atomic force microscopy (AFM) height images of a)-c): Pristine electrospun PAN fibres, where the measured diameters are consistent as measured with the SEM shown previously; d)-f): PAniNR@PAN fibrous electrodes morphologies showing increased roughness after the PAniNR growth. 1757x967mm (96 x 96 DPI)



a) Cyclic voltammograms at 10 mV/s of PAniNR@PAN fibre electrodes with varying HCl environment during the polymerization process, with additional comparison to the pure PAN fiber electrode; b) A comparison of gravimetric specific capacitance values of PAniNR@PAN fibre electrode to that of the PAni powder electrodes, with variations in HCl concentrations; both of which are measured at 10 mV/s scan rates; c) Electrochemical impedance spectroscopy (EIS) of the PAniNR@PAN electrodes fabricated at different HCl conditions; d) A comparison of PAniNR@PAN fibre electrode rate capabilities. 496x374mm (200 x 200 DPI)


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Electrochemical analysis and characterization of PAniNR@PAN nanofibers using 1M H2SO4 aqueous electrolyte, fabricated with 1M HCl acidic environment: a) The variation in CV diagrams with different scanning rates from 5mV/s to 100mV/s; b) Galvanostatic charge/discharge curves; and c) Galvanostatic charge/discharge cycling capabilities of the PAniNR@PAN nanofiber structures. 708x189mm (200 x 200 DPI)



Sensing capability of the PAniNR@PAN electrode and EC cell: a) Piezoresistive response of PAniNR@PAN nanofibers to compressive stress with a constant applied rate (inset: resistance change in response to tensile stress); b) Piezoresistive response of PAniNR@PAN nanofibers to bending; c) Piezoresistive response of the EC cell to compressive stress with a constant applied rate; d) Time response of the nanofibers to compressive stress indicating a linear behaviour; e) Piezoresistive response of the nanofibers to compressive cyclic loading; and f) Schematic of the sensing mechanism of the nanofibers. 780x429mm (200 x 200 DPI)


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a) The piezoresistive response of the PAniNR@PAN during finger tapping of the fully functioning EC cell, the peaks are increases in the resistance measurements that correspond to the tapping stresses; b) The setup used for when monitoring the piezoresistive response of the fully assembled EC cell, note the gap in the current collector on the sensing electrode side, which is used to expose the electrode material to the applied pressure; c) The measurement setup used for this test, which includes a sourcemeter for monitoring. 1370x916mm (96 x 96 DPI)



a) The resistance change resulting from mechanical bending of the PAniNR@PAN EC cell, arrows showing the time instants when the bending occurred; b) A comparison for the EC cell in a bent state vs. in a flat nonbent state. 1182x940mm (96 x 96 DPI)


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Self-Assembled Nanorod Structures on Nanofibers ... - ACS Publications

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